Sizing an electric motor
Sizing an electric motor
(OP)
Hi,
I am a Junior in ME and currently working as a graduate assistant at a wind tunnel research facility. I have been given the task of designing a mechanism which would remove a long pitot probe from the flow stream in the wind tunnel. The mechanism will be able to move a 21 in. long 5/16 in. diameter hollow metal tube 90 degrees (if you would like a diagram to better understand this, I can do that; I have already made up a CAD file that shows the scale motion). The wind tunnel will run at a maximum velocity of 70 m/s and I have calculated that the maximum torque on the pitot will be 9.36 in*lb (1.05 N*m, 149.8 oz*in) and that whatever motor I use must be able to supply this holding torque (plus about a 30% safety factor). Ideally, the operation of the motor will be done through the program LabView, and the graduates informed me that I have a 10V supply to work from.
At this point, I have run through the entire beginning to mid level design process stages, and I am looking at different motors to suit the application. Right now I am leaning toward using a stepper motor since I believe that a DC motor with encoder and feedback loop would be too complex. It is also necessary that for good pitot measurements, the pitot tube should be able to maintain its angular position +/- 1-2 degrees...the stepper motor, I think, should have no problem with this.
I am really interested in hearing what motors other people think might work better and especially WHERE reliable motor suppliers can be found. This is only my second exposure to electric motors (I took a Mechatronics class this past semester where we learned C and how to use the Dragon 12-Plus to run dc motors, steppers, and ultimately make a robot).
Thanks for the input in advance!
Jeff
I am a Junior in ME and currently working as a graduate assistant at a wind tunnel research facility. I have been given the task of designing a mechanism which would remove a long pitot probe from the flow stream in the wind tunnel. The mechanism will be able to move a 21 in. long 5/16 in. diameter hollow metal tube 90 degrees (if you would like a diagram to better understand this, I can do that; I have already made up a CAD file that shows the scale motion). The wind tunnel will run at a maximum velocity of 70 m/s and I have calculated that the maximum torque on the pitot will be 9.36 in*lb (1.05 N*m, 149.8 oz*in) and that whatever motor I use must be able to supply this holding torque (plus about a 30% safety factor). Ideally, the operation of the motor will be done through the program LabView, and the graduates informed me that I have a 10V supply to work from.
At this point, I have run through the entire beginning to mid level design process stages, and I am looking at different motors to suit the application. Right now I am leaning toward using a stepper motor since I believe that a DC motor with encoder and feedback loop would be too complex. It is also necessary that for good pitot measurements, the pitot tube should be able to maintain its angular position +/- 1-2 degrees...the stepper motor, I think, should have no problem with this.
I am really interested in hearing what motors other people think might work better and especially WHERE reliable motor suppliers can be found. This is only my second exposure to electric motors (I took a Mechatronics class this past semester where we learned C and how to use the Dragon 12-Plus to run dc motors, steppers, and ultimately make a robot).
Thanks for the input in advance!
Jeff





RE: Sizing an electric motor
An air cylinder is the first thing that comes to mind.
A stepper could do it, if its holding torque is high enough. Note that steppers typically take a half-step when you turn the power off completely, so you might run it at reduced current on two coils to hold it, and/or arrange some cooling, or deal with trying to make it lay against the stop magnetically when you turn it off, which may be too interesting.
For feedback you need two limit switches, to say {deployed | in transit | stowed}. Think of them as a two-bit encoder.
A DC motor could do the same thing, arranging the 'stowed' limit switch to shut off the motor,
and the 'deployed' switch to reduce the motor current to just enough to hold the probe in position,
against a mechanical stop that you would shim to set the deployed position to any precision you like.
Maybe you can find enough leakage flow to cool the motor, or let it hang into the tunnel a little.
If the DC motor heating bothers you, arrange it to run a quarter of a four-position Geneva wheel, so it locks mechanically against the mechanical stop and you don't need holding current to keep it there.
Mike Halloran
Pembroke Pines, FL, USA
RE: Sizing an electric motor
Keep the leadscrew pitch fairly fine, so it resists any tendency to reverse drive. Short the motor terminals using a relay to achieve a rapid stop once you reach the desired position and have removed the power. Depending on how rapidly you need to deploy and retract the pitot tube, you could use a reduction gearbox prior to the leadscrew which would increase the resistance to reverse driving, reduce the motor power required, and allow greater positional accuracy.
RE: Sizing an electric motor
Thanks for the insight!
Jeff
RE: Sizing an electric motor
I forgot to mention that I really like the Geneva wheel idea. I'm going to look into that some more!
Thanks!
Jeff
RE: Sizing an electric motor
Thanks Scotty!
Jeff
RE: Sizing an electric motor
You can use the 'deployed' limit switch to reduce the drive current by switching in a resistor, and keep the probe jammed against a mechanical stop, which through even a low ratio worm gear, should work okay. You reverse the polarity and remove the resistor from the circuit to get it unjammed and going the other way. You can do the whole thing with relays, which are easier to troubleshoot when things don't quite work as you planned.
Mike Halloran
Pembroke Pines, FL, USA
RE: Sizing an electric motor
The air drive supply - if you have none available - need only be a $200.00 off-the-shelf 120 volt compressor and hose from any hardware store.
Also. Don't be afraid to just drive it manually from outside the tunnel.
RE: Sizing an electric motor
RE: Sizing an electric motor
I was talking with the grad student the other day and he says that the main idea is to get away from the pneumatically operated system. Basically he wants a whole new electric motor system which will operate the pitot. Also, the problem with manually operating the mechanism by crude linkages from outside is that our available ports into the chamber are already stuffed with instrumentation lines, and our current setup would make it near impossible to have a rigid link attachment (its also about a 10 ft. run to the pitot from the ports with multiple obstacles in the way).
At this point, in my mind, it's being narrowed down to one of the following:
1. Stepper Motor
- Pros: With the correct holding torque, it will be able to maintain the angle exactly.
- Cons: Increased complexity and cost (need a motor driver and, I think, some sort of micro-controller), not to mention a programming code. Question: Would I need a motor driver and micro-controller for any mechanism I choose that involves a motor?
2. Geneva Wheel (http://www.youtube.com/watch?v=t7ZnHDdIvn0)
-Pros: Exact 90 degree turn with guaranteed positioning of angle (cannot move once at 90 degree position). Also, the motor no longer is the device providing the reaction torque to keep the pitot in the same place. That job now rests on the physical contact between the two rotating members (which is guaranteed to work). Lower complexity (a dumb DC motor with a low rpm)and generally lower cost.
-Cons: Hard to find/buy a geneva wheel, but could be a good opportunity for me to head to the machine shop and learn to make the parts. Also, the DC motor must turn one way, then turn back the other way to make the Geneva wheel turn both directions. This should not be too difficult (either command it to turn for a certain time, or use limit switches like suggested by some of the responses).
Question: Is it a bad idea to run a brushed motor in one direction, stop it, and then run it in the opposite direction? I have heard that the brushes are angled in a certain direction and should only be rotated one way. I have also heard that they also make straight perpendicular brushes. Any help on this would be great!
3. Geared DC Motor or DC Motor turning a worm (leadscrew) which turns a larger gear - suggested previously by ScottyUK.
-Pros: For the worm mechanism, the motor does not have to supply a holding torque during the time in the wind tunnel because the worm prevents back-driving. The geared DC motor should not require any holding torque either since it will have a large gear ratio. Lower complexity and cost and readily available.
-Cons: Can't really think of any.
Thanks again guys for your help.
Jeff
RE: Sizing an electric motor
Half a degree pretty much requires a mechanical stop of some kind no matter how you move the probe.
Mike Halloran
Pembroke Pines, FL, USA
RE: Sizing an electric motor
RE: Sizing an electric motor
Keith Cress
kcress - http://www.flaminsystems.com
RE: Sizing an electric motor
Just another cheap way to think about.
Roy
RE: Sizing an electric motor
LionelHutz and roydm: This question has been plaguing me since the beginning of this project, the answer to which was never taught in my Introduction to Electrical Engineering class. Is it possible/safe to run a 12V motor off of a 10V system? Does it not interfere with the operation of the motor? (not to mention that the torque rating will be lower and I would have to design around that too)
RE: Sizing an electric motor